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Effects of Sn addition on the microstructure and mechanical properties of as-cast, rolled and annealed Mg–4Zn alloys
Materials Science & Engineering A 585 (2013) 139–148
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Effects of Sn addition on the microstructure and mechanical properties of as-cast, rolled and annealed Mg–4Zn alloys Shanghai Wei n, Tianping Zhu, Michael Hodgson, Wei Gao Department of Chemical & Materials Engineering, Faculty of Engineering, The University of Auckland, Auckland 1142, New Zealand
art ic l e i nf o
a b s t r a c t
Article history: Received 15 April 2013 Received in revised form 3 July 2013 Accepted 8 July 2013 Available online 31 July 2013
The effects of Sn addition on the microstructure and mechanical properties of as-cast, as rolled and subsequent isothermal annealed Mg–4Zn alloys are investigated. Tensile and Rockwell hardness tests are conducted to evaluate the mechanical properties of the as-cast alloys. Improvements of the mechanical properties in Sn-containing alloys are attributed to the increasing volume fraction of secondary phases and modifying the type of intermetallic phases. Additions of Sn increase the twinning density and modify the annealing texture in as-rolled Mg–4Zn alloys. In comparing rolled-annealed Mg–4Zn to Mg–4Zn–3Sn alloy, numerous small particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangularshaped Mg2Sn particles with a size of about 175 nm. The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273 MPa, 159 MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Mg–Zn alloys Sn addition Tensile properties Rolling deformation Texture
1. Introduction Magnesium (Mg) and its alloys are the lightest engineering metals. Using Mg alloys to make vehicle parts, for example, engine block, steering wheel and transmission casting, can largely reduce the weight of the vehicle [1]. However, the application of Mg alloys is limited due to its poor ductility and formability at room temperature. Great efforts have been made to improve the ductility of Mg alloys, including refining the grain size and modifying the texture of Mg alloys through severe plastic deformation processes [2–6] and adding alloying elements [7–12]. Mg–Zn alloys containing 4–9 wt% Zn, can be ageing hardened response due to the precipitation of transition phases [13–17]. Semimetal Sn was considered as an alloying element modifying the microstructure of Mg–Zn alloys. Formation of the high melting point intermetallic compound of Mg2Sn can improve the mechanical properties of Mg alloys at elevated temperature [18–23]. The study on the effect of Zn content on the microstructure and mechanical properties of indirect extruded Mg–5Sn alloys have shown that the volumes of Mg2Sn and MgZn fine particles increase markedly as the Zn content increases [24]. Cohen et al. [25] have optimized the Mg– Zn–Sn composition for best combination of mechanical properties and structural stability at elevated temperature. Alloying elements of Ca and Si are added into Mg–Zn–Sn alloys for precipitation hardening [26]. On the other hand, Mg–Sn–Zn alloys have been reported as a high-strength heat-treatable wrought alloy system [27–29]. Sasaki
n
Corresponding author. Tel.: +64 9 3737599x88503; fax: +64 9 3737463. E-mail address: [email protected] (S. Wei).
et al. [29] have developed a new extrudable high-strength alloy, Mg– 2.2Sn–0.5Zn–1.0Al (at%), which can be extruded at 250 1C with a high yield strength of 308 MPa in tension and 280 MPa in compression. Son et al. have studied the effects of Al and Zn additions on the mechanical properties of a Mg–Sn alloy [30], and also investigated the effects of Ag additions on the microstructure and mechanical properties of Mg–6Zn–2Sn–1Ag–0.4Mn-based alloys [31]. The UTS and elongation of alloys containing 1 wt% Ag were 352 MPa and 19%, respectively. The effects of Zn/Sn mass ration [32] and minor Ca addition [33] on the microstructure and mechanical properties of Mg–Zn–Sn–Al-based alloys have also been studied. Minor Ca additions can effectively refine both grains and grain-boundary compounds, and Ca additions improved the strength of an Mg–Zn–Sn–Al alloy while sacrificing the plasticity. Although many studies have investigated the effects of alloying elements on the microstructure and creep resistance of as-cast and extruded Mg–Sn–Zn alloys, the effects of the Sn content on microstructure, texture and mechanical properties of rolled and subsequent annealed Mg–Zn alloys are not clear. This study investigated the microstructure, and mechanical properties of ascast, rolled and annealed Mg–4Zn alloys. Furthermore, the effects of Sn alloying on twinning, texture evolution and tensile fracture mechanism of Mg–4Zn alloys were examined and discussed.
2. Experiments The nominal and analyzed compositions (in wt%) of three starting materials are Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are listed in Table 1. Pure metals of Mg, Zn and Sn
0921-5093/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.07.051
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Table 1 Chemical compositions (wt%) of the experimental alloys. Nominal composition (wt%)
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
Analyzed composition (wt%) Zn
Sn
Mg
3.75 3.32 3.90
0 0.594 2.33
Bal. Bal. Bal.
(purity 4 99.9%) were melted in a mild steel crucible under Ar gas protection at 720 1C. The melt was stirred well in order to homogenize the molten alloy prior to pouring it into a steel mold being preheated to 250 1C. The cavity dimension of the steel mold is 20 mm 120 mm 80 mm. The as-cast samples were homogenized at 330 1C for 90 h, followed by cold water quenching. In order to dissolve high melting point phases of Mg2Sn, Mg–4Zn–1Sn and Mg–4Zn–3Sn, the alloys were further solid solution treated at 440 1C for 2 and 4 h. The samples were encapsulated in a quartz tube furnace which filled with high-purity Ar during heat treatment. The samples were quenched in water immediately after solid solution treatment. Plates of 5 mm thick were cut from the solid-solution treated samples and heated to 320 1C for 20 min, then rolled via multi-pass rolling with 4% reduction each pass till total 80% thickness reductions was reached. The rolling mill was operated at room temperature, and the rolled specimens were annealed at 320 1C for 15 min after each pass. Upon the completion of multipass rolling, the specimens were divided into two groups. One group of specimens was air cooled to room temperature without further treatment, which is denoted by “as-rolled specimens”. Another group of specimens was further isothermally annealed at 400 1C for 20 min, and then cooled in air to room temperature, which is denoted “as rolled-annealed specimens”. Metallographic specimens of as-cast, as-rolled, and as rolledannealed alloys were prepared following standard procedure of grinding, polishing, and etching (5 mL acetic acid, 6 g picric acid, 10 mL H2O, and 100 mL ethanol) to reveal their microstructures. The microstructures were examined using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM). The phases in the alloys and the lattice parameter was analyzed using X-ray diffractometry (Philips PW 1710, V¼40 kV, I¼40 mA) with Cu K-α X-ray source with 2θ range from 151 to 851 at a scanning rate of 0.041/s. The lattice parameter was calculated based on three strongest peaks. ImageJ software was used to estimate the volume fraction of intermetallic compounds. The volume fraction of twins in the as-rolled alloys was calculated according to point counting technique based on ASTM E562-02 standard. About fifteen digital micrographs were taken on each as-rolled sample at magnifications ranging from 100 to 200 , which is depended on the sizes of the twins. Then, a 15 point by 21 point grid was superimposed on the images. The volume fraction of twin was determined by calculating the ratio of the number of points located within twins to the total number of points. The grain size was measured using the linear intercept method according to the ASTM E112-88 standard, with at least 300 intercepts counted for each sample. Pole figures of specimens were obtained using an EDAX-TSL EBSD system and OIM 5.2 software equipped with FEI Quanta 200F Environmental Scanning Electron Microscope (ESEM). TEM observations were conducted on a Philips CM12 TEM at 120 kV. The samples for TEM observations were cut from block specimens and mechanically ground to 0.08–0.1 mm thick. Discs of 3 mm in diameter were punched from the thin foil, and finally thinned
by twin jet electro-polishing using a solution of 8.3 g LiCl, 18.6 g Mg(ClO4)2, 833.4 mL methanol, and 166.7 mL 2-butoxy-ethanol at approximately 35 1C with 90 V. The tensile test on specimens with gauge dimension of 25 mm 6 mm 2 mm ( 1 mm for as-rolled and annealed specimens) was conducted at room temperature according to the European Standard EN10002-1. The crosshead speed was 0.5 mm/min (strain rate of 3.3 10 4/s). The tensile properties presented later were the average of four testing results. Hardness measurements were carried out on a Rockwell hardness tester with the H scale (1/8 in. ball and total load is 60 kgf). The hardness result is the average of measurements at least five points on each sample.
3. Results 3.1. Microstructure and mechanical properties of as-cast alloys Optical images in Fig. 1 show the microstructures of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. All as-cast alloys exhibit a typical dendritic microstructure, which comprise α-Mg dendrites and intermetallic compounds at interdendritic regions. It can be seen from Fig. 1 that the segregation of solute elements are more obvious with increasing Sn content, and the dendrite arm spacing (DAS) is slightly reduced with the addition of Sn. The Mg–Zn–Sn ternary phase diagram has been experimentally studied by a few researchers since 1933 [34–39], and no ternary phase has been reported in the system. The X-ray diffraction patterns of three as-cast alloys are shown in Fig. 2. The low intensity peaks around the 2θ1 from 401 to 451 correspond to Mg4Zn7 intermetallic compounds. There are no peaks for Mg2Sn intermetallic phase in the Mg–4Zn–1Sn cast alloy, but it was detected when the Sn content increased to 3 wt%. The solubility of Sn in Mg matrix drops dramatically from 14.85 wt% at the eutectic temperature to 0.45 wt% at 200 1C according to Mg–Sn binary phase diagram. Thus, only a small amount of the element Sn could exist in solid solution in the Mg matrix, and the excess Sn formed Mg2Sn phase. Mg–4Zn cast alloy comprises three phases: α-Mg, a small amount of eutectic phase (α-Mg+Mg4Zn7) and Mg7Zn3 intermetallic phase [40]. The intermetallic phases in as-cast Mg–4Zn alloy present two different morphologies, irregular shape and globular shape. Similar morphologies of intermetallic particles were observed in Mg–4Zn–3Sn cast alloy, such as three particles of A, B, and C pointed by arrows in Fig. 3. The irregular shaped particle A (see Fig. 3b) presents a typical feature of partially divorced eutectic compound with alternating bright and dark regions, which is similar to the globular-shaped particle observed in Mg–4Zn alloy. High Zn content in particle A (see EDS spectrum in Fig. 3c) suggests that particle A should be eutectic phase, i.e. (α-Mg +Mg4Zn7) phase. Meanwhile, particle C should be the Mg2Sn compound due to its high Sn content (see EDS spectrum in Fig. 3g). A globular-shaped particle B (Fig. 3e) shows combined features of particle A at the right hand half and particle C at the left hand half. Distributions of Mg, Zn and Sn elements across the particle C were measured by EDS line scan and are shown in Fig. 3d. High Sn content at the left side and high Zn at the right side of the particle suggest the joining of Mg2Sn with partially divorced (α-Mg+Mg4Zn7). The morphologies of secondary phases observed in this study, are different from those reported in Mg–5Sn–5Zn cast alloy [22], which may be due to the different alloy compositions and casting parameters. The EDS results of particle C, also show that there is a small amount of Zn solutes in Mg2Sn phase. This coincides with the results reported by Godecke [35], who have studied the Mg–Sn–Zn phase diagram in the Mg2Sn–MgZn2–Zn–Sn
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Fig. 2. XRD spectra of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys.
+Mg4Zn7)+Mg2Sn), and Mg2Sn particle containing a small amount of Zn solute. The mechanical properties of three cast alloys are listed in Table 2. In general, the effect of Sn on the mechanical properties of Mg–4Zn cast alloy is not significant. It can be seen that with the addition of 3 wt% of Sn in the as-cast Mg–4Zn alloy, the UTS increased from 213 MPa to 230 MPa, YS slightly increased from 65.5 MPa to 66.7 MPa, and the elongation slightly decreased from 14% to 11%. Hardness of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn were measured by Rockwell H hardness scale, and the results are exhibited in Table 2. With the addition of Sn, the value of Rockwell hardness slightly increases from 74 to 80. 3.2. Microstructure of as-rolled and rolled-annealed alloys
Fig. 1. Optical microstructures of (a) Mg–4Zn, (b) Mg–4Zn–1Sn and (c) Mg–4Zn– 3Sn alloys in as-cast state.
area and found that solubility of Zn in Mg2Sn along the Mg2Sn– MgZn2 section is about 0.2 mass% and about 0.1 mass% along the Mg2Sn–Zn section. The solubility of Sn in MgZn2 phase has also been reported by Gladyshevsky [36] and Godecke [35]. However, Mingolo et al. [37,38] and Sirkin et al. [39] have suggested that Sn does not have any solubility in Mg7Zn3 and Mg4Zn7 compound. Thus, the microstructure of as-cast Mg–4Zn–3Sn alloys consists of αMg, eutectic phase (α-Mg+Mg4Zn7), globular-shaped particle ((α-Mg
As-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys were solid solution treated before rolling. The as-cast dendrite structure becomes homogeneous after solid solution treatment, which is shown in Fig. 4. Most of the intermetallic compounds dissolved and only a small amount of compounds (dark points in Fig. 4) remained after solid solution treatment, which is more obvious for the alloy with a high Sn content. These undissolved particles were distributed quite uniformly inside the grains and at the grain boundaries. Solid-solutionized samples were heated to 320 1C and then multi-pass rolled via a cold roller until 80% of total thickness reduction was achieved. Optical micrographs in Fig. 5 shows the microstructure of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys observed on the plane along the rolling direction (RD) and the transverse direction (TD). It can be seen that the initial as-homogenized microstructure were altered by rolling process due to the dynamic recrystallization (DRX) and static recrystallization that occurred during deformation. The as-rolled alloys exhibit finer grains as compared to the alloys after solid solution treatment. Large amounts of twinning are also observed in as-rolled Mg–4Zn alloy. With the addition of Sn, the general features of microstructure are almost the same as Mg–4Zn alloy. The difference is that Sn-containing alloys exhibit finer grains and more twinning in the grains. The volume fraction of twining in as-rolled Mg–4Zn increases from 31% to 41% with adding of 1 wt% of Sn. With Sn content further increases to 3 wt%, the volume fraction of twining increases to 47%.
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Fig. 3. (a) SEM image showing the microstructure of as-cast Mg–4Zn–3Sn alloy. (b, e, and f) high magnification of micrograph showing the three different types of intermetallic phase. (c and g) EDS spectrums and elemental compositions analyzing results of intermetallic particles A and C in (b) and (f), respectively. (d) EDS line scan showing the distribution of Mg, Zn and Sn elements of particle B. Table 2 Mechanical properties of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. Alloys
0.2% YS (MPa)
UTS (MPa)
Ef (%)
Rockwell hardness (HRC)
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
65.5 7 3.6 65.7 7 1.7 66.77 2.2
2147 6.6 2197 3.9 230 7 7.2
14 75.4 12.3 71.9 11.1 74.3
74.357 1.5 74.9 7 1.1 80.32 7 1.2
After annealing at 400 1C for 20 min, the as-rolled microstructure of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys were replaced by a nearly fully recrystallized microstructure, which are shown in Fig. 5(b), (d) and (f). The microstructures of the rolled-annealed Mg–4Zn and Mg–4Zn–1Sn sheets exhibit equiaxed grains, while the rolled-annealed Mg–4Zn–3Sn alloy shows bimodal structure. The bimodality of microstructure comprises grain sizes from 11 μm to 76 μm, suggesting that the recrystallization characteristics were not uniform. Measurements of grain size shows that addition of 1 wt% Sn caused a slight decrease in grain size of the rolled-annealed Mg–4Zn alloy from 31 μm to 25 μm. The average grain size of rolled-annealed Mg–4Zn–3Sn alloy is 24 μm, similar to that of Mg–4Zn–1Sn
alloy, indicating the limited grain refining effect by increasing the content of Sn. Precipitation occurring during the rolling process is another microstructural features. Fig. 6 exhibits the XRD patterns of solid solution treated and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn samples. It can be seen that only α-Mg peaks is present in all solid solution treated samples, and Mg2Sn peaks are seen in rolled-annealed Mg–4Zn–3Sn alloys. The presence of the Mg2Sn phase indicates that the precipitation occurred during rolling process from the supersaturated Mg matrix. Wei et al. [33] also reported that the diffraction peak intensity of Mg2Sn is increased after hot rolling, indicating the occurrence of massive precipitation of Mg2Sn during hot rolling deformation of Mg–4.5Zn–4.5Sn–2Al–0.2Ca alloy. Bright-field TEM images in Fig. 7 show the microstructures of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys. It can be seen that almost no particles appeared in the rolled-annealed Mg–4Zn alloy. Meanwhile, numerous tiny particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles of 175 nm. The rectangular shaped Mg2Sn phase is clearly different from the reported rod-shaped Mg2Sn particles observed in the aged Mg–Sn–Zn alloy
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recrystallization. In contrast, coarse particles tend to stimulate recrystallization nucleation in their immediate vicinities. In this study, the bimodal grain structure of rolled-annealed Mg–4Zn–3Sn may be related to the coexistence of nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles. Further study is needed to understand the mechanisms involved in the annealing behavior of Sn added Mg–Zn alloys. 3.3. Tensile properties of as-rolled and rolled-annealed alloys Table 3 summaries the tensile properties of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. It can be seen that the rolling deformation significantly improves the tensile strength of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys while sacrificing the ductility. After rolling deformation, the UTS of as-rolled Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn are about 29%, 31% and 45% higher than the as-cast alloys. The most significantly improvement is the yield strength (YS). The YS of asrolled Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are 241, 251 and 305 MPa, 267%, 282% and 357% higher than the as-cast alloys. However, the elongations of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn– 3Sn alloys reduce from 14%, 12% and 11% in as-cast state to 3.0%, 6.1% and 5.5% in as-rolled state, respectively. The high tensile strengths of as-rolled alloys are mainly attributed to the residual work-hardening and grain refinement effect. Generally, Mg alloys with finer grain size should be more ductile. However, grain refining effect on the ductility might be compromised by the residual work-hardening, as evidenced by the heavily twinned structure. Compared with the as-rolled samples, the tensile strengths of rolled-annealed samples decreased, but their ductility is improved significantly. The elongations of rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are 17.6%, 19.3%, and 16.2%, respectively. The dramatically change in tensile strength and ductility of annealed Mg–4Zn and Mg–4Zn–3Sn alloys is as expected due to the recovery and dynamic crystallization during isothermal annealing treatment. The as-rolled microstructure contains a large amount of twins, was replaced by fully recrystallized fine grains in rolled-annealed sample.
4. Discussion 4.1. Effects of Sn on as-cast Mg–4Zn alloys
Fig. 4. Optical micrographs of (a) Mg–4Zn, (b) Mg–4Zn–1Sn and (c) Mg–4Zn–3Sn alloys after solid solution treatment.
[20]. The volume fraction of these small particles tends to increase with the increase of Sn content. The result is consistent with the finding reported in the extruded Mg–Sn–Zn–Al alloy [29] and indirect-extruded Mg–5Sn–xZn (x ¼1, 2, 4) alloys [24]. The particles presented in an alloy not only affect the annealing behavior of the alloy which depends on the volume fraction, size, shape of particles, and interparticle distance [41], but also influences the deformation behavior of the alloy. Even dispersions of fine particles can homogenize plastic deformation and retard the
According to the Mg–Sn binary phase diagram, the solubility of Sn in Mg drops dramatically from 14.5 wt% at the eutectic temperature to nearly zero at room temperature. However, supersaturated Sn may still be dissolved in α-Mg due to the nonequilibrium solidification during casting. In this study, no Mg2Sn phase was detected in the Mg–4Zn–1Sn cast alloy by XRD, which indicates that 1 wt% Sn is probably dissolved in α-Mg completely. The microstructure of Mg–4Zn cast alloy was altered significantly by adding 3 wt% Sn due to the formation of Mg2Sn particles with different morphologies, as shown in Fig. 4. Chen et al. [21] have reported that a small addition of Sn in Mg–Zn–Al alloy contributed to the formation of the dispersed Mg2Sn particles and improvement of strength at ambient and elevated-temperature. In the present study, the volume fraction of intermetallic phases in Mg– 4Zn cast alloy increased from 0.85% to 1.4% with addition of 3 wt% Sn. The discrete Mg2Sn particles distributed mainly within dendrites and along grain boundaries, which can act as pins inhibiting the motion of dislocations during tensile deformation. Hence, the strength and hardness of as-cast Mg–4Zn alloy were enhanced by addition of Sn. Furthermore, solid solution of a small amount of Sn in α-Mg also strengthened the alloys.
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Fig. 5. Optical micrographs showing the microstructure of the as-rolled and rolled-annealed alloys: (a and b) Mg–4Zn, (c, d) Mg–4Zn–1Sn, and (e and f) Mg–4Zn–3Sn. RD and TD in the figure are the rolling direction and transverse direction.
4.2. Effects of Sn on as-rolled and rolled-annealed Mg–4Zn alloys 4.2.1. Microstructure Alloying element Sn plays an important role in refining the recrystallized grains and increasing the twinning density after deformation. The increased twinning density is likely attributed to the strong effect of Sn on the stacking fault energy of Mg. Muzyk et al. [42] calculated the generalized stacking fault energy of Mg and Mg alloys based on the density function theory, and found that an addition of Sn reduces the stacking fault energy of Mg significantly. It was also found that the formation energy of twins in Mg is lowered by Sn additions. Furthermore, it was reported that the grain boundary provides favorable site and plays an important role in the nucleation of twinning [43]. The finer grain size of Sn-containing alloys compared to the as-rolled Sn-free Mg–4Zn alloy (see Fig. 5) may promote the twinning nucleation. Thus, higher volume fraction of twinning formed in Sn added Mg–4Zn alloy. After 20 min annealing at 400 1C, both Mg–4Zn and Sn-containing alloys were fully recrystallized. The recrystallization texture of rolledannealed Mg–4Zn and Mg–4Zn–1Sn alloys was measured using EBSD. Fig. 8 shows the inverse pole figures (IPF), (0002) and (1120) pole figures of the rolled-annealed Mg–4Zn and Mg–4Zn–1Sn alloys on transverse sections (rolling direction normal to the sample plane).
Fig. 6. XRD patterns of the as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys after solid solution treatment, and the rolled alloys after annealing at 400 1C for 20 min.
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significantly increased to 22 after addition of 1 wt% Sn. The color distribution in the IPF maps (Fig. 8a and b) shows noticeable differences of grain orientation in Mg–4Zn and Mg–4Zn–1Sn samples. The amount of red colored grains in Mg–4Zn–1Sn (Fig. 8b) is much higher than that in Mg–4Zn (Fig. 8a), suggesting that Sn addition in Mg–4Zn alloy could enhance its texture. Stanford et al. [44] have also found that the addition of Sn has a tendency to sharpen the extrusion texture compared to pure Mg. This is different from the effect of 0.5% Sr addition in Mg–Zn alloy, which significantly weakened the texture of the alloy [45]. A number of mechanisms have been proposed for texture modification, including particle-stimulated nucleation (PSN), twin-induced nucleation (TIN) and axial (c/a) ratio changes [46]. In this study, tiny particles in Sn-containing Mg–4Zn alloys (Fig. 7) are likely too small to activate PSN. However, it has been reported that change of axial (c/a) ratio has a significantly effect on the activation of non-basal slip system and critical resolved shear stress (CRSS) [47]. Wang et al. [48] summarized the effects of axial (c/a) ratio on the texture of hexagonal structural materials. Metals with c/a rations approximately equal to the ideal c/a ratio of 1.633 tend to form basal fiber textures. When the axial ratio c/a 41.633, the basal pole rotated from normal direction (ND) to rolling direction (RD) to form double peaks. On the other hand, the basal pole rotated toward the transverse direction (TD) to develop double peaks when the c/a ratio o 1.633. Kim et al. [49] also investigated the effect of axial ratio on texture development. The texture of Mg–Zn alloy which has not much change in the axial ratio exhibited a very similar basal rolling texture as pure Mg. While the Mg–Al alloy with increased c/a ratio showed an increased intensity of basal pole, and the Mg–Li alloy with reduced axial ratio exhibited reduced intensity of basal pole. In the present study, the value of c/a ration of Mg–4Zn slightly increases from 1.6221 to 1.6268 with an addition of 1 wt% Sn. Therefore, enhanced intensity of texture along basal plane for Mg–4Zn–1Sn alloy may be due to the increase of its axial ratio.
Fig. 7. TEM micrographs showing the fine particles in the rolled-annealed alloys: (a) Mg–4Zn and (b) Mg–4Zn–3Sn.
Table 3 Tensile Properties of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg– 4Zn–3Sn alloys. Alloys
0.2% YS (MPa)
UTS (MPa)
Ef (%)
As-rolled
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
241 70.1 251 71.8 305 71.4
276 74.7 286 71.7 334 70.8
3 72.2 6.1 72.4 5.5 73.3
Rolled-annealed
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
104 76.6 117 73.5 159 78.2
235 72.8 260 713 273 73.5
17.6 70.6 19.3 70.35 16.2 72.5
Both alloys exhibit the basal texture clearly that most of grains were oriented with their basal planes parallel to the rolling direction. The peak intensity of rolled-annealed Mg–4Zn sample is 7.5, which
4.2.2. Tensile properties and fracture analysis Fig. 9 shows the tensile stress–strain curves of as-cast, as-rolled and rolled-annealed Mg–4Zn (three dash curves) and Mg–4Zn–3Sn alloys (three solid curves). It can be seen that the strengths of as-rolled and rolled-annealed Mg–4Zn alloy were significantly enhanced with addition of Sn. However, the effect of Sn on the ductility varied with processing conditions. The ductility of as-rolled Mg–4Zn–3Sn alloy slightly increased comparing with as-rolled Mg–4Zn alloy. Meanwhile, addition of Sn slightly decreased the ductility of rolled-annealed Mg–4Zn alloy. The existence of discretely distributed fine particles in Sn-containing alloy could retard the motion of dislocations. Thus, the significantly improved strength of Mg–4Zn–3Sn alloy comparing with Mg–4Zn alloy is mainly resulted from the dispersion strengthening of nanosized Zn-rich particles and rectangular Mg2Sn particles. SEM images in Fig. 10 show the tensile fractured surfaces of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys. The rolledannealed Mg–4Zn alloy exhibits typical cleavage pattern with a small amount of dimples on fracture surface, i.e., quasi-cleavage. For Mg–4Zn–3Sn alloy, cleavage facets are still evident, but with much finer scale than Mg–4Zn alloy (as shown in Fig. 10(c) and (f)). In addition, many dimples appeared on the fracture surface (Fig. 10 (e)). Nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles were observed within most of these dimples. It is understood that twinning and intermetallic particles are two key factors determining the fracture modes of Mg alloys during plastic deformation [50]. Three deformation modes: basal slip, prismatic slip and {1012} twinning, can be observed during plastic deformation of Mg at room temperature [51]. The basal slip and {1012}
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Fig. 8. EBSD analysis and texture plots of (a) and (c) the rolled-annealed Mg–4Zn and (b) and (d) rolled-annealed Mg–4Zn–1Sn. The maximum values of texture intensities for Mg–4Zn and Mg–4Zn–1Sn are 7.5 and 22.1, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
rolled-annealed Mg–4Zn–3Sn alloy also provide the preferential site for the void nucleation. Generally, the voids nucleate after a few percent of plastic deformation, and the final separation may occur around 25% of deformation [53]. When a cleavage crack interacts with the void formed around fine particles, the growth rate of crack may accelerate. Thus, the fracture regime of rolledannealed Mg–4Zn–3Sn alloy exhibits a quasi-cleavage with a large amount of dimples.
5. Conclusion The microstructure and mechanical properties of as-cast, asrolled and subsequently isothermal annealed Mg–4Zn, Mg–4Zn– 1Sn and Mg–4Zn–3Sn alloys were studied. Following conclusions can be drawn.
Fig. 9. Tensile strain–stress curves of the as-cast, as-rolled and subsequent annealed Mg–4Zn and Mg–4Zn–3Sn alloys.
twinning are relatively easy to activate in Mg, while prismatic system requires 40–50 MPa to yield [51]. In the room temperature tensile deformation of Mg–4Zn alloy, basal slip and twinning are relatively easy to activate. When a propagating twin interacts with grain boundaries, high stress concentration may developed in a nearby region of the adjacent grain at which twin nucleation or crack initiations may take place [52]. With further increased load, the crack propagates through the whole grains. This fracture mechanism should also apply to the tensile deformed rolled-annealed Mg–4Zn–3Sn alloy. However, the fine particles in
(1) The additions of Sn significantly affect the volume fraction of secondary phases and the type of intermetallic phases in ascast Mg–4Zn alloy. The microstructure of as-cast Mg–4Zn–3Sn alloys consists of α-Mg, eutectic phase (α-Mg+Mg4Zn7), globular-shaped phase (Mg4Zn7+Mg2Sn), and Mg2Sn phase containing a small amount of Zn solutes. As a result of Sn additions, the tensile strength and Rockwell hardness were improved, while the ductility slightly decreased. (2) Additions of Sn in Mg–4Zn alloys resulted in the refined recrystallized grains, increased twinning density and modified annealing texture. Compared to annealed Mg–4Zn alloy, numerous small particles are dispersed in the matrix of annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanostructured Zn–rich particles and rectangular-shaped of the Mg2Sn particles with size about 175 nm.
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Fig. 10. SEM fractographic images of rolled-annealed alloys: (a–c) Mg–4Zn and (d–f) Mg–4Zn–3Sn alloys. (b, c) and (e, f) are the high magnification images of (a) and (d), respectively.
(3) The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273 MPa, 159 MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy. The fracture mechanism of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys are quasicleavage fracture, but smaller size of cleavage facets and enhanced dimples are exhibited in the Sn containing alloy.
[12] [13] [14] [15]
Acknowledgment
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of Sn addition on the microstructure and mechanical properties of as-cast, rolled and annealed Mg–4Zn alloys Shanghai Wei n, Tianping Zhu, Michael Hodgson, Wei Gao Department of Chemical & Materials Engineering, Faculty of Engineering, The University of Auckland, Auckland 1142, New Zealand
art ic l e i nf o
a b s t r a c t
Article history: Received 15 April 2013 Received in revised form 3 July 2013 Accepted 8 July 2013 Available online 31 July 2013
The effects of Sn addition on the microstructure and mechanical properties of as-cast, as rolled and subsequent isothermal annealed Mg–4Zn alloys are investigated. Tensile and Rockwell hardness tests are conducted to evaluate the mechanical properties of the as-cast alloys. Improvements of the mechanical properties in Sn-containing alloys are attributed to the increasing volume fraction of secondary phases and modifying the type of intermetallic phases. Additions of Sn increase the twinning density and modify the annealing texture in as-rolled Mg–4Zn alloys. In comparing rolled-annealed Mg–4Zn to Mg–4Zn–3Sn alloy, numerous small particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangularshaped Mg2Sn particles with a size of about 175 nm. The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273 MPa, 159 MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Mg–Zn alloys Sn addition Tensile properties Rolling deformation Texture
1. Introduction Magnesium (Mg) and its alloys are the lightest engineering metals. Using Mg alloys to make vehicle parts, for example, engine block, steering wheel and transmission casting, can largely reduce the weight of the vehicle [1]. However, the application of Mg alloys is limited due to its poor ductility and formability at room temperature. Great efforts have been made to improve the ductility of Mg alloys, including refining the grain size and modifying the texture of Mg alloys through severe plastic deformation processes [2–6] and adding alloying elements [7–12]. Mg–Zn alloys containing 4–9 wt% Zn, can be ageing hardened response due to the precipitation of transition phases [13–17]. Semimetal Sn was considered as an alloying element modifying the microstructure of Mg–Zn alloys. Formation of the high melting point intermetallic compound of Mg2Sn can improve the mechanical properties of Mg alloys at elevated temperature [18–23]. The study on the effect of Zn content on the microstructure and mechanical properties of indirect extruded Mg–5Sn alloys have shown that the volumes of Mg2Sn and MgZn fine particles increase markedly as the Zn content increases [24]. Cohen et al. [25] have optimized the Mg– Zn–Sn composition for best combination of mechanical properties and structural stability at elevated temperature. Alloying elements of Ca and Si are added into Mg–Zn–Sn alloys for precipitation hardening [26]. On the other hand, Mg–Sn–Zn alloys have been reported as a high-strength heat-treatable wrought alloy system [27–29]. Sasaki
n
Corresponding author. Tel.: +64 9 3737599x88503; fax: +64 9 3737463. E-mail address: [email protected] (S. Wei).
et al. [29] have developed a new extrudable high-strength alloy, Mg– 2.2Sn–0.5Zn–1.0Al (at%), which can be extruded at 250 1C with a high yield strength of 308 MPa in tension and 280 MPa in compression. Son et al. have studied the effects of Al and Zn additions on the mechanical properties of a Mg–Sn alloy [30], and also investigated the effects of Ag additions on the microstructure and mechanical properties of Mg–6Zn–2Sn–1Ag–0.4Mn-based alloys [31]. The UTS and elongation of alloys containing 1 wt% Ag were 352 MPa and 19%, respectively. The effects of Zn/Sn mass ration [32] and minor Ca addition [33] on the microstructure and mechanical properties of Mg–Zn–Sn–Al-based alloys have also been studied. Minor Ca additions can effectively refine both grains and grain-boundary compounds, and Ca additions improved the strength of an Mg–Zn–Sn–Al alloy while sacrificing the plasticity. Although many studies have investigated the effects of alloying elements on the microstructure and creep resistance of as-cast and extruded Mg–Sn–Zn alloys, the effects of the Sn content on microstructure, texture and mechanical properties of rolled and subsequent annealed Mg–Zn alloys are not clear. This study investigated the microstructure, and mechanical properties of ascast, rolled and annealed Mg–4Zn alloys. Furthermore, the effects of Sn alloying on twinning, texture evolution and tensile fracture mechanism of Mg–4Zn alloys were examined and discussed.
2. Experiments The nominal and analyzed compositions (in wt%) of three starting materials are Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are listed in Table 1. Pure metals of Mg, Zn and Sn
0921-5093/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.07.051
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Table 1 Chemical compositions (wt%) of the experimental alloys. Nominal composition (wt%)
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
Analyzed composition (wt%) Zn
Sn
Mg
3.75 3.32 3.90
0 0.594 2.33
Bal. Bal. Bal.
(purity 4 99.9%) were melted in a mild steel crucible under Ar gas protection at 720 1C. The melt was stirred well in order to homogenize the molten alloy prior to pouring it into a steel mold being preheated to 250 1C. The cavity dimension of the steel mold is 20 mm 120 mm 80 mm. The as-cast samples were homogenized at 330 1C for 90 h, followed by cold water quenching. In order to dissolve high melting point phases of Mg2Sn, Mg–4Zn–1Sn and Mg–4Zn–3Sn, the alloys were further solid solution treated at 440 1C for 2 and 4 h. The samples were encapsulated in a quartz tube furnace which filled with high-purity Ar during heat treatment. The samples were quenched in water immediately after solid solution treatment. Plates of 5 mm thick were cut from the solid-solution treated samples and heated to 320 1C for 20 min, then rolled via multi-pass rolling with 4% reduction each pass till total 80% thickness reductions was reached. The rolling mill was operated at room temperature, and the rolled specimens were annealed at 320 1C for 15 min after each pass. Upon the completion of multipass rolling, the specimens were divided into two groups. One group of specimens was air cooled to room temperature without further treatment, which is denoted by “as-rolled specimens”. Another group of specimens was further isothermally annealed at 400 1C for 20 min, and then cooled in air to room temperature, which is denoted “as rolled-annealed specimens”. Metallographic specimens of as-cast, as-rolled, and as rolledannealed alloys were prepared following standard procedure of grinding, polishing, and etching (5 mL acetic acid, 6 g picric acid, 10 mL H2O, and 100 mL ethanol) to reveal their microstructures. The microstructures were examined using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM). The phases in the alloys and the lattice parameter was analyzed using X-ray diffractometry (Philips PW 1710, V¼40 kV, I¼40 mA) with Cu K-α X-ray source with 2θ range from 151 to 851 at a scanning rate of 0.041/s. The lattice parameter was calculated based on three strongest peaks. ImageJ software was used to estimate the volume fraction of intermetallic compounds. The volume fraction of twins in the as-rolled alloys was calculated according to point counting technique based on ASTM E562-02 standard. About fifteen digital micrographs were taken on each as-rolled sample at magnifications ranging from 100 to 200 , which is depended on the sizes of the twins. Then, a 15 point by 21 point grid was superimposed on the images. The volume fraction of twin was determined by calculating the ratio of the number of points located within twins to the total number of points. The grain size was measured using the linear intercept method according to the ASTM E112-88 standard, with at least 300 intercepts counted for each sample. Pole figures of specimens were obtained using an EDAX-TSL EBSD system and OIM 5.2 software equipped with FEI Quanta 200F Environmental Scanning Electron Microscope (ESEM). TEM observations were conducted on a Philips CM12 TEM at 120 kV. The samples for TEM observations were cut from block specimens and mechanically ground to 0.08–0.1 mm thick. Discs of 3 mm in diameter were punched from the thin foil, and finally thinned
by twin jet electro-polishing using a solution of 8.3 g LiCl, 18.6 g Mg(ClO4)2, 833.4 mL methanol, and 166.7 mL 2-butoxy-ethanol at approximately 35 1C with 90 V. The tensile test on specimens with gauge dimension of 25 mm 6 mm 2 mm ( 1 mm for as-rolled and annealed specimens) was conducted at room temperature according to the European Standard EN10002-1. The crosshead speed was 0.5 mm/min (strain rate of 3.3 10 4/s). The tensile properties presented later were the average of four testing results. Hardness measurements were carried out on a Rockwell hardness tester with the H scale (1/8 in. ball and total load is 60 kgf). The hardness result is the average of measurements at least five points on each sample.
3. Results 3.1. Microstructure and mechanical properties of as-cast alloys Optical images in Fig. 1 show the microstructures of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. All as-cast alloys exhibit a typical dendritic microstructure, which comprise α-Mg dendrites and intermetallic compounds at interdendritic regions. It can be seen from Fig. 1 that the segregation of solute elements are more obvious with increasing Sn content, and the dendrite arm spacing (DAS) is slightly reduced with the addition of Sn. The Mg–Zn–Sn ternary phase diagram has been experimentally studied by a few researchers since 1933 [34–39], and no ternary phase has been reported in the system. The X-ray diffraction patterns of three as-cast alloys are shown in Fig. 2. The low intensity peaks around the 2θ1 from 401 to 451 correspond to Mg4Zn7 intermetallic compounds. There are no peaks for Mg2Sn intermetallic phase in the Mg–4Zn–1Sn cast alloy, but it was detected when the Sn content increased to 3 wt%. The solubility of Sn in Mg matrix drops dramatically from 14.85 wt% at the eutectic temperature to 0.45 wt% at 200 1C according to Mg–Sn binary phase diagram. Thus, only a small amount of the element Sn could exist in solid solution in the Mg matrix, and the excess Sn formed Mg2Sn phase. Mg–4Zn cast alloy comprises three phases: α-Mg, a small amount of eutectic phase (α-Mg+Mg4Zn7) and Mg7Zn3 intermetallic phase [40]. The intermetallic phases in as-cast Mg–4Zn alloy present two different morphologies, irregular shape and globular shape. Similar morphologies of intermetallic particles were observed in Mg–4Zn–3Sn cast alloy, such as three particles of A, B, and C pointed by arrows in Fig. 3. The irregular shaped particle A (see Fig. 3b) presents a typical feature of partially divorced eutectic compound with alternating bright and dark regions, which is similar to the globular-shaped particle observed in Mg–4Zn alloy. High Zn content in particle A (see EDS spectrum in Fig. 3c) suggests that particle A should be eutectic phase, i.e. (α-Mg +Mg4Zn7) phase. Meanwhile, particle C should be the Mg2Sn compound due to its high Sn content (see EDS spectrum in Fig. 3g). A globular-shaped particle B (Fig. 3e) shows combined features of particle A at the right hand half and particle C at the left hand half. Distributions of Mg, Zn and Sn elements across the particle C were measured by EDS line scan and are shown in Fig. 3d. High Sn content at the left side and high Zn at the right side of the particle suggest the joining of Mg2Sn with partially divorced (α-Mg+Mg4Zn7). The morphologies of secondary phases observed in this study, are different from those reported in Mg–5Sn–5Zn cast alloy [22], which may be due to the different alloy compositions and casting parameters. The EDS results of particle C, also show that there is a small amount of Zn solutes in Mg2Sn phase. This coincides with the results reported by Godecke [35], who have studied the Mg–Sn–Zn phase diagram in the Mg2Sn–MgZn2–Zn–Sn
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Fig. 2. XRD spectra of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys.
+Mg4Zn7)+Mg2Sn), and Mg2Sn particle containing a small amount of Zn solute. The mechanical properties of three cast alloys are listed in Table 2. In general, the effect of Sn on the mechanical properties of Mg–4Zn cast alloy is not significant. It can be seen that with the addition of 3 wt% of Sn in the as-cast Mg–4Zn alloy, the UTS increased from 213 MPa to 230 MPa, YS slightly increased from 65.5 MPa to 66.7 MPa, and the elongation slightly decreased from 14% to 11%. Hardness of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn were measured by Rockwell H hardness scale, and the results are exhibited in Table 2. With the addition of Sn, the value of Rockwell hardness slightly increases from 74 to 80. 3.2. Microstructure of as-rolled and rolled-annealed alloys
Fig. 1. Optical microstructures of (a) Mg–4Zn, (b) Mg–4Zn–1Sn and (c) Mg–4Zn– 3Sn alloys in as-cast state.
area and found that solubility of Zn in Mg2Sn along the Mg2Sn– MgZn2 section is about 0.2 mass% and about 0.1 mass% along the Mg2Sn–Zn section. The solubility of Sn in MgZn2 phase has also been reported by Gladyshevsky [36] and Godecke [35]. However, Mingolo et al. [37,38] and Sirkin et al. [39] have suggested that Sn does not have any solubility in Mg7Zn3 and Mg4Zn7 compound. Thus, the microstructure of as-cast Mg–4Zn–3Sn alloys consists of αMg, eutectic phase (α-Mg+Mg4Zn7), globular-shaped particle ((α-Mg
As-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys were solid solution treated before rolling. The as-cast dendrite structure becomes homogeneous after solid solution treatment, which is shown in Fig. 4. Most of the intermetallic compounds dissolved and only a small amount of compounds (dark points in Fig. 4) remained after solid solution treatment, which is more obvious for the alloy with a high Sn content. These undissolved particles were distributed quite uniformly inside the grains and at the grain boundaries. Solid-solutionized samples were heated to 320 1C and then multi-pass rolled via a cold roller until 80% of total thickness reduction was achieved. Optical micrographs in Fig. 5 shows the microstructure of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys observed on the plane along the rolling direction (RD) and the transverse direction (TD). It can be seen that the initial as-homogenized microstructure were altered by rolling process due to the dynamic recrystallization (DRX) and static recrystallization that occurred during deformation. The as-rolled alloys exhibit finer grains as compared to the alloys after solid solution treatment. Large amounts of twinning are also observed in as-rolled Mg–4Zn alloy. With the addition of Sn, the general features of microstructure are almost the same as Mg–4Zn alloy. The difference is that Sn-containing alloys exhibit finer grains and more twinning in the grains. The volume fraction of twining in as-rolled Mg–4Zn increases from 31% to 41% with adding of 1 wt% of Sn. With Sn content further increases to 3 wt%, the volume fraction of twining increases to 47%.
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Fig. 3. (a) SEM image showing the microstructure of as-cast Mg–4Zn–3Sn alloy. (b, e, and f) high magnification of micrograph showing the three different types of intermetallic phase. (c and g) EDS spectrums and elemental compositions analyzing results of intermetallic particles A and C in (b) and (f), respectively. (d) EDS line scan showing the distribution of Mg, Zn and Sn elements of particle B. Table 2 Mechanical properties of as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. Alloys
0.2% YS (MPa)
UTS (MPa)
Ef (%)
Rockwell hardness (HRC)
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
65.5 7 3.6 65.7 7 1.7 66.77 2.2
2147 6.6 2197 3.9 230 7 7.2
14 75.4 12.3 71.9 11.1 74.3
74.357 1.5 74.9 7 1.1 80.32 7 1.2
After annealing at 400 1C for 20 min, the as-rolled microstructure of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys were replaced by a nearly fully recrystallized microstructure, which are shown in Fig. 5(b), (d) and (f). The microstructures of the rolled-annealed Mg–4Zn and Mg–4Zn–1Sn sheets exhibit equiaxed grains, while the rolled-annealed Mg–4Zn–3Sn alloy shows bimodal structure. The bimodality of microstructure comprises grain sizes from 11 μm to 76 μm, suggesting that the recrystallization characteristics were not uniform. Measurements of grain size shows that addition of 1 wt% Sn caused a slight decrease in grain size of the rolled-annealed Mg–4Zn alloy from 31 μm to 25 μm. The average grain size of rolled-annealed Mg–4Zn–3Sn alloy is 24 μm, similar to that of Mg–4Zn–1Sn
alloy, indicating the limited grain refining effect by increasing the content of Sn. Precipitation occurring during the rolling process is another microstructural features. Fig. 6 exhibits the XRD patterns of solid solution treated and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn samples. It can be seen that only α-Mg peaks is present in all solid solution treated samples, and Mg2Sn peaks are seen in rolled-annealed Mg–4Zn–3Sn alloys. The presence of the Mg2Sn phase indicates that the precipitation occurred during rolling process from the supersaturated Mg matrix. Wei et al. [33] also reported that the diffraction peak intensity of Mg2Sn is increased after hot rolling, indicating the occurrence of massive precipitation of Mg2Sn during hot rolling deformation of Mg–4.5Zn–4.5Sn–2Al–0.2Ca alloy. Bright-field TEM images in Fig. 7 show the microstructures of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys. It can be seen that almost no particles appeared in the rolled-annealed Mg–4Zn alloy. Meanwhile, numerous tiny particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles of 175 nm. The rectangular shaped Mg2Sn phase is clearly different from the reported rod-shaped Mg2Sn particles observed in the aged Mg–Sn–Zn alloy
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recrystallization. In contrast, coarse particles tend to stimulate recrystallization nucleation in their immediate vicinities. In this study, the bimodal grain structure of rolled-annealed Mg–4Zn–3Sn may be related to the coexistence of nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles. Further study is needed to understand the mechanisms involved in the annealing behavior of Sn added Mg–Zn alloys. 3.3. Tensile properties of as-rolled and rolled-annealed alloys Table 3 summaries the tensile properties of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys. It can be seen that the rolling deformation significantly improves the tensile strength of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys while sacrificing the ductility. After rolling deformation, the UTS of as-rolled Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn are about 29%, 31% and 45% higher than the as-cast alloys. The most significantly improvement is the yield strength (YS). The YS of asrolled Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are 241, 251 and 305 MPa, 267%, 282% and 357% higher than the as-cast alloys. However, the elongations of Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn– 3Sn alloys reduce from 14%, 12% and 11% in as-cast state to 3.0%, 6.1% and 5.5% in as-rolled state, respectively. The high tensile strengths of as-rolled alloys are mainly attributed to the residual work-hardening and grain refinement effect. Generally, Mg alloys with finer grain size should be more ductile. However, grain refining effect on the ductility might be compromised by the residual work-hardening, as evidenced by the heavily twinned structure. Compared with the as-rolled samples, the tensile strengths of rolled-annealed samples decreased, but their ductility is improved significantly. The elongations of rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys are 17.6%, 19.3%, and 16.2%, respectively. The dramatically change in tensile strength and ductility of annealed Mg–4Zn and Mg–4Zn–3Sn alloys is as expected due to the recovery and dynamic crystallization during isothermal annealing treatment. The as-rolled microstructure contains a large amount of twins, was replaced by fully recrystallized fine grains in rolled-annealed sample.
4. Discussion 4.1. Effects of Sn on as-cast Mg–4Zn alloys
Fig. 4. Optical micrographs of (a) Mg–4Zn, (b) Mg–4Zn–1Sn and (c) Mg–4Zn–3Sn alloys after solid solution treatment.
[20]. The volume fraction of these small particles tends to increase with the increase of Sn content. The result is consistent with the finding reported in the extruded Mg–Sn–Zn–Al alloy [29] and indirect-extruded Mg–5Sn–xZn (x ¼1, 2, 4) alloys [24]. The particles presented in an alloy not only affect the annealing behavior of the alloy which depends on the volume fraction, size, shape of particles, and interparticle distance [41], but also influences the deformation behavior of the alloy. Even dispersions of fine particles can homogenize plastic deformation and retard the
According to the Mg–Sn binary phase diagram, the solubility of Sn in Mg drops dramatically from 14.5 wt% at the eutectic temperature to nearly zero at room temperature. However, supersaturated Sn may still be dissolved in α-Mg due to the nonequilibrium solidification during casting. In this study, no Mg2Sn phase was detected in the Mg–4Zn–1Sn cast alloy by XRD, which indicates that 1 wt% Sn is probably dissolved in α-Mg completely. The microstructure of Mg–4Zn cast alloy was altered significantly by adding 3 wt% Sn due to the formation of Mg2Sn particles with different morphologies, as shown in Fig. 4. Chen et al. [21] have reported that a small addition of Sn in Mg–Zn–Al alloy contributed to the formation of the dispersed Mg2Sn particles and improvement of strength at ambient and elevated-temperature. In the present study, the volume fraction of intermetallic phases in Mg– 4Zn cast alloy increased from 0.85% to 1.4% with addition of 3 wt% Sn. The discrete Mg2Sn particles distributed mainly within dendrites and along grain boundaries, which can act as pins inhibiting the motion of dislocations during tensile deformation. Hence, the strength and hardness of as-cast Mg–4Zn alloy were enhanced by addition of Sn. Furthermore, solid solution of a small amount of Sn in α-Mg also strengthened the alloys.
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Fig. 5. Optical micrographs showing the microstructure of the as-rolled and rolled-annealed alloys: (a and b) Mg–4Zn, (c, d) Mg–4Zn–1Sn, and (e and f) Mg–4Zn–3Sn. RD and TD in the figure are the rolling direction and transverse direction.
4.2. Effects of Sn on as-rolled and rolled-annealed Mg–4Zn alloys 4.2.1. Microstructure Alloying element Sn plays an important role in refining the recrystallized grains and increasing the twinning density after deformation. The increased twinning density is likely attributed to the strong effect of Sn on the stacking fault energy of Mg. Muzyk et al. [42] calculated the generalized stacking fault energy of Mg and Mg alloys based on the density function theory, and found that an addition of Sn reduces the stacking fault energy of Mg significantly. It was also found that the formation energy of twins in Mg is lowered by Sn additions. Furthermore, it was reported that the grain boundary provides favorable site and plays an important role in the nucleation of twinning [43]. The finer grain size of Sn-containing alloys compared to the as-rolled Sn-free Mg–4Zn alloy (see Fig. 5) may promote the twinning nucleation. Thus, higher volume fraction of twinning formed in Sn added Mg–4Zn alloy. After 20 min annealing at 400 1C, both Mg–4Zn and Sn-containing alloys were fully recrystallized. The recrystallization texture of rolledannealed Mg–4Zn and Mg–4Zn–1Sn alloys was measured using EBSD. Fig. 8 shows the inverse pole figures (IPF), (0002) and (1120) pole figures of the rolled-annealed Mg–4Zn and Mg–4Zn–1Sn alloys on transverse sections (rolling direction normal to the sample plane).
Fig. 6. XRD patterns of the as-cast Mg–4Zn, Mg–4Zn–1Sn and Mg–4Zn–3Sn alloys after solid solution treatment, and the rolled alloys after annealing at 400 1C for 20 min.
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significantly increased to 22 after addition of 1 wt% Sn. The color distribution in the IPF maps (Fig. 8a and b) shows noticeable differences of grain orientation in Mg–4Zn and Mg–4Zn–1Sn samples. The amount of red colored grains in Mg–4Zn–1Sn (Fig. 8b) is much higher than that in Mg–4Zn (Fig. 8a), suggesting that Sn addition in Mg–4Zn alloy could enhance its texture. Stanford et al. [44] have also found that the addition of Sn has a tendency to sharpen the extrusion texture compared to pure Mg. This is different from the effect of 0.5% Sr addition in Mg–Zn alloy, which significantly weakened the texture of the alloy [45]. A number of mechanisms have been proposed for texture modification, including particle-stimulated nucleation (PSN), twin-induced nucleation (TIN) and axial (c/a) ratio changes [46]. In this study, tiny particles in Sn-containing Mg–4Zn alloys (Fig. 7) are likely too small to activate PSN. However, it has been reported that change of axial (c/a) ratio has a significantly effect on the activation of non-basal slip system and critical resolved shear stress (CRSS) [47]. Wang et al. [48] summarized the effects of axial (c/a) ratio on the texture of hexagonal structural materials. Metals with c/a rations approximately equal to the ideal c/a ratio of 1.633 tend to form basal fiber textures. When the axial ratio c/a 41.633, the basal pole rotated from normal direction (ND) to rolling direction (RD) to form double peaks. On the other hand, the basal pole rotated toward the transverse direction (TD) to develop double peaks when the c/a ratio o 1.633. Kim et al. [49] also investigated the effect of axial ratio on texture development. The texture of Mg–Zn alloy which has not much change in the axial ratio exhibited a very similar basal rolling texture as pure Mg. While the Mg–Al alloy with increased c/a ratio showed an increased intensity of basal pole, and the Mg–Li alloy with reduced axial ratio exhibited reduced intensity of basal pole. In the present study, the value of c/a ration of Mg–4Zn slightly increases from 1.6221 to 1.6268 with an addition of 1 wt% Sn. Therefore, enhanced intensity of texture along basal plane for Mg–4Zn–1Sn alloy may be due to the increase of its axial ratio.
Fig. 7. TEM micrographs showing the fine particles in the rolled-annealed alloys: (a) Mg–4Zn and (b) Mg–4Zn–3Sn.
Table 3 Tensile Properties of as-rolled and rolled-annealed Mg–4Zn, Mg–4Zn–1Sn and Mg– 4Zn–3Sn alloys. Alloys
0.2% YS (MPa)
UTS (MPa)
Ef (%)
As-rolled
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
241 70.1 251 71.8 305 71.4
276 74.7 286 71.7 334 70.8
3 72.2 6.1 72.4 5.5 73.3
Rolled-annealed
Mg–4Zn Mg–4Zn–1Sn Mg–4Zn–3Sn
104 76.6 117 73.5 159 78.2
235 72.8 260 713 273 73.5
17.6 70.6 19.3 70.35 16.2 72.5
Both alloys exhibit the basal texture clearly that most of grains were oriented with their basal planes parallel to the rolling direction. The peak intensity of rolled-annealed Mg–4Zn sample is 7.5, which
4.2.2. Tensile properties and fracture analysis Fig. 9 shows the tensile stress–strain curves of as-cast, as-rolled and rolled-annealed Mg–4Zn (three dash curves) and Mg–4Zn–3Sn alloys (three solid curves). It can be seen that the strengths of as-rolled and rolled-annealed Mg–4Zn alloy were significantly enhanced with addition of Sn. However, the effect of Sn on the ductility varied with processing conditions. The ductility of as-rolled Mg–4Zn–3Sn alloy slightly increased comparing with as-rolled Mg–4Zn alloy. Meanwhile, addition of Sn slightly decreased the ductility of rolled-annealed Mg–4Zn alloy. The existence of discretely distributed fine particles in Sn-containing alloy could retard the motion of dislocations. Thus, the significantly improved strength of Mg–4Zn–3Sn alloy comparing with Mg–4Zn alloy is mainly resulted from the dispersion strengthening of nanosized Zn-rich particles and rectangular Mg2Sn particles. SEM images in Fig. 10 show the tensile fractured surfaces of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys. The rolledannealed Mg–4Zn alloy exhibits typical cleavage pattern with a small amount of dimples on fracture surface, i.e., quasi-cleavage. For Mg–4Zn–3Sn alloy, cleavage facets are still evident, but with much finer scale than Mg–4Zn alloy (as shown in Fig. 10(c) and (f)). In addition, many dimples appeared on the fracture surface (Fig. 10 (e)). Nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles were observed within most of these dimples. It is understood that twinning and intermetallic particles are two key factors determining the fracture modes of Mg alloys during plastic deformation [50]. Three deformation modes: basal slip, prismatic slip and {1012} twinning, can be observed during plastic deformation of Mg at room temperature [51]. The basal slip and {1012}
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Fig. 8. EBSD analysis and texture plots of (a) and (c) the rolled-annealed Mg–4Zn and (b) and (d) rolled-annealed Mg–4Zn–1Sn. The maximum values of texture intensities for Mg–4Zn and Mg–4Zn–1Sn are 7.5 and 22.1, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
rolled-annealed Mg–4Zn–3Sn alloy also provide the preferential site for the void nucleation. Generally, the voids nucleate after a few percent of plastic deformation, and the final separation may occur around 25% of deformation [53]. When a cleavage crack interacts with the void formed around fine particles, the growth rate of crack may accelerate. Thus, the fracture regime of rolledannealed Mg–4Zn–3Sn alloy exhibits a quasi-cleavage with a large amount of dimples.
5. Conclusion The microstructure and mechanical properties of as-cast, asrolled and subsequently isothermal annealed Mg–4Zn, Mg–4Zn– 1Sn and Mg–4Zn–3Sn alloys were studied. Following conclusions can be drawn.
Fig. 9. Tensile strain–stress curves of the as-cast, as-rolled and subsequent annealed Mg–4Zn and Mg–4Zn–3Sn alloys.
twinning are relatively easy to activate in Mg, while prismatic system requires 40–50 MPa to yield [51]. In the room temperature tensile deformation of Mg–4Zn alloy, basal slip and twinning are relatively easy to activate. When a propagating twin interacts with grain boundaries, high stress concentration may developed in a nearby region of the adjacent grain at which twin nucleation or crack initiations may take place [52]. With further increased load, the crack propagates through the whole grains. This fracture mechanism should also apply to the tensile deformed rolled-annealed Mg–4Zn–3Sn alloy. However, the fine particles in
(1) The additions of Sn significantly affect the volume fraction of secondary phases and the type of intermetallic phases in ascast Mg–4Zn alloy. The microstructure of as-cast Mg–4Zn–3Sn alloys consists of α-Mg, eutectic phase (α-Mg+Mg4Zn7), globular-shaped phase (Mg4Zn7+Mg2Sn), and Mg2Sn phase containing a small amount of Zn solutes. As a result of Sn additions, the tensile strength and Rockwell hardness were improved, while the ductility slightly decreased. (2) Additions of Sn in Mg–4Zn alloys resulted in the refined recrystallized grains, increased twinning density and modified annealing texture. Compared to annealed Mg–4Zn alloy, numerous small particles are dispersed in the matrix of annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanostructured Zn–rich particles and rectangular-shaped of the Mg2Sn particles with size about 175 nm.
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Fig. 10. SEM fractographic images of rolled-annealed alloys: (a–c) Mg–4Zn and (d–f) Mg–4Zn–3Sn alloys. (b, c) and (e, f) are the high magnification images of (a) and (d), respectively.
(3) The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273 MPa, 159 MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy. The fracture mechanism of rolled-annealed Mg–4Zn and Mg–4Zn–3Sn alloys are quasicleavage fracture, but smaller size of cleavage facets and enhanced dimples are exhibited in the Sn containing alloy.
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Acknowledgment
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